This invention relates to a thermal stencil master sheet for stencil printing, and more particularly, it relates to a thermal stencil master sheet obtained by adhering a thermoplastic resin film onto a porous substrate with an adhesive.
Thermal stencil master sheets provide stencils when placed in contact with a thermal print head of a thermal transfer printer or the equivalent thereof. The print head is activated as the stencil master sheet is scrolled by, creating perforations therein. These perforations permit the flow of ink from a mesh printing drone to a receiving substrate, creating a printed image that conforms to the stencil pattern.
Representative disclosures in the art of thermal stencil master sheets are as follows:
U.S. Pat. No. 4,628,813 issued to Hasegawa et al. describes a stencil duplicator which prepares a stencil master and prints with the stencil master. The means for making the stencil master includes apparatus for selective illumination with light.
U.S. Pat. No. 4,961,377 issued to Bando et al. describes a thermal stencil master sheet which employs a urethane adhesive.
U.S. Pat. No. 5,160,564 issued to Hasegawa et al. describes a process for producing a thermal stencil master sheet which protects the film from breakage by laminating thermoplastic films onto the surfaces of a base film, bonding porous substrates to the thermoplastic films and subsequently stripping the thermoplastic films with the porous substrates from the base film.
U.S. Pat. No. 5,245,932 issued to Ujiie describes a thermal stencil master sheet of allegedly high resolution through the use of different zones (contacting zones and clearance zones) on the thermoplastic resin.
U.S. Pat. No. 5,373,785 issued to Yamamoto et al. describes a mimeo graphic transfer printing machine, wherein ink is forced through the stencil supported on a stencil supporting drum.
U.S. Pat. No. 5,415,090 issued to Natori et al. describes a method for manufacturing a print master by forming perforations in a thermosensitive stencil paper.
U.S. Pat. No. 5,438,347 issued to Shishido et al. describes a device for making a thermal stencil master sheet within a stencil printer.
U.S. Pat. No. 5,450,789 issued to Hasegawa describes a stencil printing method which uses a plurality of stencil master plates which can be superimposed during printing to achieve color printing.
U.S. Pat. No. 5,513,565 issued to Hasegawa describes a stencil printing device with a plurality of printing drums for printing full color images.
U.S. Pat. No. 5,517,913 issued to Oshio et al. describes a stencil printing device which uses a sensor for detecting an ink type.
U.S. Pat. No. 5,522,313 and U.S. Pat. No. 5,243,906, both issued to Okusawa describes thermal stencil master plates, wherein an unprocessed portion includes swelled and solidified lumps of thermoplastic resin film said to avoid the expansion of the perforations.
U.S. Pat. No. 5,526,032 issued to Nakamura describes a method for processing a stencil master plate using a thermal head.
The thermal stencil master sheets are typically obtained by laminating a thermoplastic resin film onto a porous substrate such as a porous thin paper with an adhesive. A release layer is typically provided on the surface of the thermoplastic resin film to prevent adhesion of the film to the receiving substrate during use. The thermoplastic resin films used for the thermal stencil master sheets have varied to a limited extent. Polyester films, films of propylene copolymers and vinylidene chloride-vinyl chloride copolymer films have been said to be suitable. The composition of the porous substrate can vary widely and is said to include the thin paper of both natural and synthetic fibers. The adhesives presently being used include vinyl acetates, acrylics and rubbers. Various deficiencies of these particular adhesives include the need for solvents and dispersing agents. These components reduce productivity by lowering line speeds due to drying and solvent elimination. Another deficiency is the poor adhesive strength exhibited by some adhesives. The adhesive has many requirements which include the following:
1. the adhesive must permit the coating process to be carried out rapidly; PA1 2. the adhesive must melt with the thermoplastic resin film when forming perforations; and PA1 3. the adhesive must be resistant to solvents within the printing ink. PA1 (i) The cationic polymerization typically has no volatile by-products which can represent health hazards and/or produce an unpleasant odor. PA1 (ii) The cationic polymerization, once activated, typically continues for some time in the absence of light (dark cure). PA1 (iii) The cationic polymerization medium is very stable in the absence of light and typically can have a shelf-life of years if stored in a light-free environment. PA1 a) forming a liquid layer between a porous substrate and a thermoplastic resin film with an adhesive formulation, and PA1 b) curing the liquid layer of the adhesive formulation to form a solid layer by exposure to UV or visible light, said adhesive formulation comprising:
The use of organic solvents complicates compliance with environmental regulations and restrictions and also adds to the cost in that the solvent removed must be captured and/or incinerated. Suitable water-based adhesives have not been achieved and the use of hot melt adhesives is expected to cause problems with the thin thermoplastic resin film.
Vinyl acetate adhesives have been widely used due to their ease of handling. However, higher adhesive strength is desired. The thermal stencil master sheets with this adhesive suffer damage when in contact with the printing ink over an extended period, resulting in unclear printed images.
Alternatives to the vinyl acetates have been proposed, such as the urethane adhesives containing a urethane prepolymer as disclosed in U.S. Pat. No. 4,961,377. These adhesives are said to take from 24-48 hours to cure at room temperature following the addition of moisture (see column 4, lines 23-26). In addition, solvents are said to be employed when necessary to obtain a uniform coating. While these urethane adhesives accelerate the laminating process, complex procedures are required to prepare and use the urethane prepolymer. It is desirable to provide thermal stencil master sheets having an adhesive which is simple to prepare and use and provides stencils with high endurance.
Ultraviolet radiation curable coatings are known and most comprise a reactive oligomer or a reactive monomer, a photoinitiator and optional additives. UV curable coatings have many uses including photoimaging inks and photoresist technology. The use of coatings with UV/visible light curing components has several advantages when compared to solvent or aqueous based coatings. For example, there is no need to dry a coating of a photo-curable formulation applied to a substrate or capture/incinerate any organic solvents. Typically, all of the coating formulation forms the final coating and there are no losses from the evaporation of solvent.
Conventional photopolymerizable coatings can cure by a cationic mechanism or by a free-radical mechanism or both depending on the monomers and initiators used. Photopolymerizable monomers and oligomers which cure by a cationic curing mechanism are preferred. The cationic curing mechanism provides the following advantages: